Alternative pre-cooling arrangement

ABSTRACT

A natural gas liquefaction system, the system comprising a first precooling refrigeration system that accepts at least a natural gas feed stream, a second precooling refrigeration system that accepts at least a first refrigerant stream; and a cryogenic heat exchanger fluidly connected to the first precooling refrigeration system and the second precooling refrigeration system that accepts the natural gas feed stream from the first precooling refrigeration system and the first refrigerant stream from the second precooling refrigeration system to liquefy the natural gas feed stream, where the second precooling refrigeration system accepts only stream(s) having a composition different from the stream(s) accepted by the first precooling refrigeration system.

BACKGROUND

The present invention relates to a system and method for liquefaction ofa gas stream, and more specifically, to a system and method forliquefaction of a natural gas stream in large capacity liquefactionplants.

Over the past few years, the liquid natural gas (LNG) industry has movedtowards using large capacity liquefaction plants to achieve favorableeconomics associated with the large plants. Scale-up problems arise,however, when refrigerant mass and volume flow rates are increased. Forexample, the design of compression equipment, particularly thecompression equipment associated with precooling, becomes problematicbecause the increased flow rates require larger compressor impellerswith higher tip speeds, thicker and heavier casings, and higher inletvelocities to the impellers. As the equipment is scaled up, the designof the compressor becomes more problematic as fundamental aerodynamiclimits are approached and, thus, the scale up may be limited by theseconsiderations. In addition these precooling compressors are large andoften contain multiple stages. Moreover, scale-up in many instancesrequires large, heavy equipment that can be difficult and costly tomanufacture and/or install.

U.S. Pat. No. 6,962,060 (Petrowski et al.) assigned to the assignee ofthe present invention, discloses one alternative system designed forliquefaction at large plants that includes a compressor systemcomprising a first compressor having a first stage and a second stagewherein the first stage of the first compressor is adapted to compress afirst gas and the second stage of the first compressor is adapted tocompress a combination of a fourth gas and an intermediate compressedgas from the first stage of the first compressor; and a secondcompressor having a first stage and a second stage wherein the firststage of the second compressor is adapted to compress a second gas andthe second stage of the second compressor is adapted to compress acombination of a third gas and an intermediate compressed gas from thefirst stage of the second compressor.

There is a need for a method and system that provides stable operationat full rates and during turndown for larger capacity liquefactionplants.

BRIEF SUMMARY

Embodiments of the present invention satisfy this need in the art byproviding a liquid natural gas liquefaction system and process that isstable and operational at full rates and during turndown for largercapacity liquefaction plants.

In one exemplary embodiment a natural gas liquefaction system isdisclosed, the system comprises: a first precooling refrigeration systemthat accepts at least a natural gas feed stream; a second precoolingrefrigeration system that accepts at least a first refrigerant stream;and a cryogenic heat exchanger fluidly connected to the first precoolingrefrigeration system and the second precooling refrigeration system thataccepts the natural gas feed stream from the first precoolingrefrigeration system and the first refrigerant stream from the secondprecooling refrigeration system to liquefy the natural gas feed stream,wherein the second precooling refrigeration system accepts onlystream(s) having a composition different from the stream(s) accepted bythe first precooling refrigeration system.

In another exemplary embodiment, a method for liquefying natural gas isdisclosed, the method comprising the steps of: providing a natural gasfeed stream; providing a first refrigerant stream; precooling in a firstprecooling refrigeration system at least the natural gas feed stream;precooling in a second precooling refrigeration system at least thefirst refrigerant stream; and vaporizing the precooled first refrigerantstream in a cryogenic heat exchanger to cool the precooled natural gasfeed stream through indirect heat exchange, wherein the secondprecooling refrigeration system precools only stream(s) having acomposition different from the stream(s) precooled by the firstprecooling refrigeration system.

In yet another exemplary embodiment, a natural gas liquefaction systemfor large capacity liquefaction plants is disclosed, the systemcomprising: a first precooling refrigeration system that accepts onestream selected from the group consisting of:

a natural gas feed stream, and an at least one refrigerant stream; asecond precooling refrigeration system that accepts any remainingstream(s) not accepted by the first precooling refrigeration system andfrom the group consisting of: the natural gas feed stream, and the atleast one refrigerant stream; and a cryogenic heat exchanger fluidlyconnected to the first precooling refrigeration system and the secondprecooling refrigeration system and adapted to accept the natural gasfeed stream and the at least one refrigerant stream from the firstprecooling refrigeration system and the second precooling refrigerationsystem, wherein the at least one refrigerant stream is used to liquefythe natural gas feed stream, wherein the second precooling refrigerationsystem accepts only stream(s) having a composition different from thestream(s) accepted by the first precooling refrigeration system.

BRIEF DESCRIPTION OF THE EXEMPLARY DRAWINGS

The foregoing brief summary, as well as the following detaileddescription of exemplary embodiments, is better understood when read inconjunction with the appended drawings. For the purpose of illustratingembodiments of the invention, there is shown in the drawings exemplaryembodiments of the invention; however, the invention is not limited tothe specific methods and instrumentalities disclosed. In the drawings:

FIG. 1 is a flow chart illustrating an exemplary system and methodinvolving aspects of the present invention;

FIG. 2A is a flow chart illustrating an exemplary system and methodinvolving aspects of the present invention;

FIG. 2B is a flow chart illustrating an exemplary system and methodinvolving aspects of the present invention;

FIG. 3 is a flow chart illustrating an exemplary system and methodinvolving aspects of the present invention;

FIG. 4 is a flow chart illustrating an exemplary system and methodinvolving aspects of the present invention;

FIG. 5 is a flow chart illustrating an exemplary system and methodinvolving aspects of the present invention;

FIG. 6 is a flow chart illustrating an exemplary system and methodinvolving aspects of the present invention;

FIG. 7A is a flow chart illustrating an exemplary system and methodinvolving aspects of the present invention;

FIG. 7B is a flow chart illustrating an exemplary system and methodinvolving aspects of the present invention;

FIG. 8A is a flow chart illustrating an exemplary system and methodinvolving aspects of the present invention; and

FIG. 8B is a flow chart illustrating an exemplary system and methodinvolving aspects of the present invention.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary embodiment of the invention as appliedto a pre-cooled refrigerant system and process. In this exemplary system100, propane is used to precool both a natural gas feed stream 102 and aliquefaction refrigerant stream 104. The natural gas feed stream 102 maybe pretreated, for example. The liquefaction refrigerant stream 104 maybe a pure or a mixed refrigerant, for example. It should be noted thatwhile the exemplary embodiments described below may refer to theliquefaction refrigerant stream as a mixed refrigerant stream, theliquefaction refrigerant stream described below may also be a purerefrigerant stream, for example. Depending on refrigerant availabilityin the local area and system requirements (e.g., adjusting thecomposition of the mixed refrigerant to match the cooling curve foroptimal cooling performance), the liquefaction refrigerant stream 104may comprise one or more of the following: nitrogen, methane, ethylene,ethane, propylene, propane, iso-butane, n-butane, and iso-pentane, forexample.

The compression of the vapor resulting from the cooling of the naturalgas feed stream 102 may occur in one compressor 118 while thecompression of the propane vapor generated from cooling of liquefactionrefrigerant stream 104 may occur in a separate compressor 126.

Precooling of the natural gas feed stream 102 and the mixed refrigerantstream 104 may be accomplished by vaporizing a precooling refrigerantsuch as propane at four different pressure levels in closed-loopprecooling refrigeration system(s). The natural gas feed stream 102 maybe precooled because of equipment limitations and for efficiencypurposes. It should be noted that while propane may be used as theprecooling refrigerant for vaporizing at four different pressure levels(as illustrated in exemplary FIGS. 1-7A), carbon dioxide, methane,propane, butane, iso-butane, propylene, ethane, ethylene, R22, HFCrefrigerants, including, but not limited to, R410A, R134A, R507, R23, orcombinations thereof, may also be used, for example.

Cooling of the natural gas feed stream 102 is performed in unit 106.Unit 106 may comprise a series of heat exchangers, valves, andseparators as illustrated in FIG. 2A. Natural gas feed stream 102 iscooled by indirect heat exchange against a precooling refrigerant in aseries of propane evaporators 202, 204, 206, 208 that may operate atsuccessively lower pressures (202 being the highest and 208 being thelowest, for example) producing cooled successive streams 203, 205, 207,and 150. The evaporation of propane at the four pressures results infour propane vapor streams 110, 112, 114, 116 that are then compressedin compressor 118. The resulting compressed stream 120 is then condensedin propane condenser 122, producing liquid stream 124 for reintroductioninto the series of propane evaporators 202, 204, 206, 208. Propanecondensers used in these types of methods and systems may include, forexample, a propane de-superheater, a condenser, an accumulator, and apropane subcooler. It should be noted that while this exemplaryembodiment illustrated in FIGS. 1, 2A, 2B, 3, 4, 5, 6, and 7A uses afour stage pre-cooling system, the pre-cooling system may comprise asingle-stage, a two-stage, a three-stage, or systems with greater thanfour stages, for example, where the series of propane evaporators mayoperate at successively lower pressures.

Cooling of the mixed refrigerant stream 104 is performed in unit 108.Unit 108 may also comprise a series of heat exchangers, valves, andseparators as illustrated in FIG. 2B. The mixed refrigerant stream 104may also be cooled by indirect heat exchange against the precoolingrefrigerant in a series of propane evaporators 222, 224, 226, 228 thatmay operate at successively lower pressures (222 being the highest and228 being the lowest, for example) producing cooled successive streams223, 225, 227, and 138. The evaporation of propane at the four pressuresresults in four propane vapor streams 130, 132, 134,136 that are thencompressed in compressor 126. The resulting compressed stream 127 isthen condensed in propane condenser 128, producing liquid stream 129 forreintroduction into the series of propane evaporators 222, 224, 226,228.

Cooled mixed refrigerant stream 138 is separated in phase separator 140into a liquid mixed refrigerant stream 142 and a vapor mixed refrigerantstream 144. Liquid mixed refrigerant stream 142 is sub-cooled in thecryogenic heat exchanger (MCHE) 146 producing stream 147. Stream 147 maythen be reduced in pressure through isenthalpic valve 148 producingstream 149. Stream 149 may then be vaporized in the shell side of theMCHE 146 to provide cooling to tubeside streams 142, 144,150.

Vapor mixed refrigerant steam 144 is condensed and sub-cooled in theMCHE 146 to produce stream 151. Stream 151 may then be reduced inpressure through isenthalpic valve 152 to produce stream 153. Stream 153may then be vaporized in the shell side of the MCHE 146 to providecooling to tubeside streams 142, 144, 150.

The cooled natural gas feed stream 150 may enter the MCHE 146 where itis further cooled producing product stream 166 that may be, for example,liquid natural gas (LNG).

Low pressure mixed refrigerant stream 145 exiting the MCHE 146 iscompressed in the low pressure mixed refrigerant compressor 154 toproduce stream 155. It should be noted that the refrigerant compressorsof all of the exemplary embodiments may include one or more intercoolersand compressor casings. For example, mixed refrigerant compressor 154may include one or more intercoolers and at least one compressor casing.Intercoolers and aftercoolers use an ambient heat sink (air or water) toreject compression heat to the environment.

Stream 155 is cooled in intercooler 156 to produce stream 157. Stream157 is further compressed in the medium pressure mixed refrigerantcompressor 158 to produce stream 159. Stream 159 is cooled inintercooler 160 to produce stream 161. Stream 161 is further compressedin high pressure mixed refrigerant compressor 162 to produce stream 163.Stream 163 is cooled in aftercooler 164 to be recycled back as originalmixed refrigerant stream 104.

The exemplary embodiment illustrated in FIG. 1 shows how the powersupplied to the refrigeration compressors 118, 126, 154, 158, 162 areprovided by two equal sized directly connected gas turbines 180, 182.For example, mixed refrigerant compressors 154, 158 are driven by gasturbine driver 180 while mixed refrigerant compressor 160 and thepropane compressors 118, 126 are driven by gas turbine driver 182. Inthis exemplary embodiment, the design pressure level between the mixedrefrigerant compressors 158 and 162 may be chosen such that the workrequired by the two gas turbine drivers 180, 182 is essentially equal.The gas turbine drivers in all exemplary embodiments may be single-shaftgas turbines or multi-shaft gas turbines, for example.

This exemplary embodiment is independent of the method used to power therefrigeration compressors 118, 126, 154, 158 and 162. The refrigerationcompressors 118, 126, 154, 158 and 162, and the refrigerationcompressors of the other exemplary embodiments may be driven by one ormore gas turbines, electric motors, steam turbines, or a combination ofdifferent drivers. As illustrated in FIG. 1, the gas turbines 180, 182may include starter/helper electric motors 184, 186 respectively toassist in starting the gas turbines 180, 182 and optimally, to provideadditional power to assist the gas turbines 180, 182, or to generatepower for exportation into the power grid when excess power is availablefrom the gas turbines. Moreover, for the exemplary embodimentillustrated in FIG. 1, and all other exemplary embodiments disclosed,the order of the compressor bodies and the starter/helper electricmotors coupled to each driver is not fixed and may bemanipulated/altered pursuant to any system requirements, maintenancerequirements, and/or plant design requirements. For example,starter/helper electric motor 186 in FIG. 1 may be positioned away fromand not adjacent to driver 182 (i.e., at the opposite end of the driverstring). The positions of the compressor bodies 118, 126, 162 may alsobe exchanged.

FIG. 3 illustrates another exemplary embodiment 300 where the propanecompressors 318, 326 are powered by different drivers 380, 382respectively. In this exemplary embodiment, the power demand from theequivalent gas turbine drivers 380, 382 may be balanced by adjustment ofthe discharge pressure of low pressure mixed refrigerant compressor 354.

As illustrated in the exemplary embodiment 300 in FIG. 3, cooling of thenatural gas feed stream 302 is performed in unit 306. Like unit 106 ofFIG. 1, unit 306 may comprise a series of heat exchangers, valves, andseparators as illustrated in FIG. 2A. Natural gas feed stream 302 iscooled by indirect heat exchange to ultimately produce cooled stream350. The evaporation of propane at the four pressures results in fourpropane vapor streams 310, 312, 314, 316 that may then be compressed incompressor 318. The resulting compressed stream 320 may then becondensed in propane condenser 322, producing liquid stream 324 forreintroduction into the series of propane evaporators as shown in FIG.2A.

Cooling of the mixed refrigerant stream 304 is performed in unit 308.Unit 308 may also comprise a series of heat exchangers, valves, andseparators as illustrated in FIG. 2B. The mixed refrigerant stream 304may also be cooled by indirect heat exchange to ultimately producecooled stream 338. The evaporation of propane at the four pressuresresults in four propane vapor streams 330, 332, 334, 336 that may thenbe compressed in compressor 326. The resulting compressed stream 327 maythen be condensed in propane condenser 328, producing liquid stream 329for reintroduction into the series of propane evaporators as shown inFIG. 2B.

Again cooled mixed refrigerant stream 338 is separated in phaseseparator 340 into a liquid mixed refrigerant stream 342 and a vapormixed refrigerant stream 344. Liquid mixed refrigerant stream 342 issub-cooled in the cryogenic heat exchanger (MCHE) 346 producing stream347. Stream 347 may then be reduced in pressure through isenthalpicvalve 348 producing stream 349. Stream 349 may then be vaporized in theshell side of the MCHE 346 to provide cooling to tubeside streams 342,344, 350.

Vapor mixed refrigerant steam 344 is condensed and sub-cooled in theMCHE 346 to produce stream 351. Stream 351 may then be reduced inpressure through isenthalpic valve 352 to produce stream 353. Stream 353may then be vaporized in the shell side of the MCHE 346 to providecooling to tubeside streams 342, 344, 350.

The cooled natural gas feed stream 350 may enter the MCHE 346 where itis further cooled producing product stream 366 that may be, for example,liquid natural gas (LNG).

Low pressure mixed refrigerant stream 345 exiting the MCHE 346 iscompressed in the low pressure refrigerant compressor 354 to producestream 355. Stream 355 is cooled in intercooler 356 to produce stream357. Stream 357 is further compressed in the high pressure refrigerantcompressor 362 to produce stream 363. Stream 363 is cooled inaftercooler 364 to be recycled back as original mixed refrigerant stream304.

Power is supplied to the refrigeration compressors 318, 326, 354, 362 bytwo equal sized directly connected gas turbines 380, 382. As illustratedin FIG. 3, the gas turbines 380, 382 may include starter/helper electricmotors 384, 386 respectively to assist in starting the gas turbines 380,382 and optimally, to provide additional power to assist the gasturbines 380, 382, or for exportation into the power grid when excesspower is available from the gas turbines.

FIG. 4 illustrates another exemplary embodiment 400 where the positionof compressors 418, 426 of FIG. 3 may be swapped such that one of thedrivers provides power to the propane compressor 418 and the highpressure refrigerant compressor 462, while the other driver providespower to the propane compressor 426 and the low pressure refrigerantcompressor 454.

As illustrated in the exemplary embodiment 400 in FIG. 4, cooling of thenatural gas feed stream 402 is performed in unit 406. Like unit 106 ofFIG. 1, unit 406 may comprise a series of heat exchangers, valves, andseparators as illustrated in FIG. 2A. Natural gas feed stream 402 iscooled by indirect heat exchange to ultimately produce cooled stream450. The evaporation of propane at the four pressures results in fourpropane vapor streams 410, 412, 414, 416 that may then be compressed incompressor 418. The resulting compressed stream 420 may then becondensed in propane condenser 422, producing liquid stream 424 forreintroduction into the series of propane evaporators as shown in FIG.2A.

Cooling of the mixed refrigerant stream 404 is performed in unit 408.Unit 408 may also comprise a series of heat exchangers, valves, andseparators as illustrated in FIG. 2B. The mixed refrigerant stream 404may also be cooled by indirect heat exchange to ultimately producecooled stream 438. The evaporation of propane at the four pressuresresults in four propane vapor streams 430, 432, 434, 436 that may thenbe compressed in compressor 426. The resulting compressed stream 427 maythen be condensed in propane condenser 428, producing liquid stream 429for reintroduction into the series of propane evaporators as shown inFIG. 2B.

Again cooled mixed refrigerant stream 438 is separated in phaseseparator 440 into a liquid mixed refrigerant stream 442 and a vapormixed refrigerant stream 444. Liquid mixed refrigerant stream 442 issub-cooled in the cryogenic heat exchanger (MCHE) 446 producing stream447. Stream 447 may then be reduced in pressure through isenthalpicvalve 448 producing stream 449. Stream 449 may then be vaporized in theshell side of the MCHE 446 to provide cooling to tubeside streams 442,444, 450.

Vapor mixed refrigerant steam 444 is condensed and sub-cooled in theMCHE 446 to produce stream 451. Stream 451 may then be reduced inpressure through isenthalpic valve 452 to produce stream 453. Stream 453may then be vaporized in the shell side of the MCHE 446 to providecooling to tubeside streams 442, 444, 450.

The cooled natural gas feed stream 450 may enter the MCHE 446 where itis further cooled producing product stream 466 that may be, for example,liquid natural gas (LNG).

Low pressure mixed refrigerant stream 445 exiting the MCHE 446 iscompressed in the low pressure refrigerant compressor 454 to producestream 455. Stream 455 is cooled in intercooler 456 to produce stream457. Stream 457 is further compressed in high pressure refrigerantcompressor 462 to produce stream 463. Stream 463 is cooled inaftercooler 464 to be recycled back as original mixed refrigerant stream404.

Power is supplied to the refrigeration compressors 418, 426, 454, 462 bytwo equal sized directly connected gas turbines 480, 482. As illustratedin FIG. 4, the gas turbines 480, 482 may include starter/helper electricmotors 484, 486 respectively to assist in starting the gas turbines 480,482 and optimally, to provide additional power to assist the gasturbines 480, 482, or for exportation into the power grid when excesspower is available from the gas turbines.

FIG. 5 illustrates yet another exemplary embodiment 500 as applied to athree loop refrigeration system. In this exemplary embodiment 500, unit506 precools a third refrigerant stream 503 in addition to the naturalgas feed stream 502. Like unit 106 of FIG. 1, unit 506 may comprise aseries of heat exchangers, valves, and separators as illustrated in FIG.2A. Natural gas feed stream 502 is cooled by indirect heat exchange toultimately produce cooled stream 550. The evaporation of propane at thefour pressures results in four propane vapor streams 510, 512, 514, 516that may then be compressed in compressor 518. The resulting compressedstream 520 may then be condensed in propane condenser 522, producingliquid stream 524 for reintroduction into the series of propaneevaporators as shown in FIG. 2A.

Cooling of the mixed refrigerant stream 504 is performed in unit 508.Unit 508 may also comprise a series of heat exchangers, valves, andseparators as illustrated in FIG. 2B. The mixed refrigerant stream 504may also be cooled by indirect heat exchange to ultimately producecooled stream 538. The evaporation of propane at the four pressuresresults in four propane vapor streams 530, 532, 534, 536 that may thenbe compressed in compressor 526. The resulting compressed stream 527 maythen be condensed in propane condenser 528, producing liquid stream 529for reintroduction into the series of propane evaporators as shown inFIG. 2B.

Cooled mixed refrigerant stream 538 is subcooled in the cryogenic heatexchanger (MCHE) 546 producing stream 547. Stream 547 may then bereduced in pressure through isenthalpic valve 548 producing stream 549.Stream 549 may then be vaporized in the shell side of the MCHE 546 toprovide cooling to tubeside streams 505, 538, and 550.

Cooled mixed refrigerant stream 505 may also be subcooled and liquefiedin MCHE 546 producing stream 569 then subcooled in exchanger 568producing stream 551. Exchanger 568 may be a wound coil type exchanger,for example. The resulting stream 551 may then be reduced in pressurethrough isenthalpic valve 552 to produce stream 553. Stream 553 may thenbe vaporized in exchanger 568 to provide refrigeration for subcoolingboth the feed gas stream (entering as stream 567 and exiting as 566) andthe third refrigerant stream 569. After vaporization and warming, thirdrefrigerant stream 553 exits exchanger 568 as stream 593 and is thencompressed by compressor 594 to produce stream 595. Stream 595 is thencooled in the mixed refrigerant intercooler 596 to produce stream 597.Stream 597 is compressed in compressor 598 to produce stream 599. Stream599 is then cooled in mixed refrigerant aftercooler 501 to be recycledback as original stream 503.

The cooled natural gas feed stream 550 may enter the MCHE 546 where itis further cooled producing stream 567. Stream 567 may then be subcooledin exchanger 568 to produce product stream 566 that may be, for example,liquid natural gas (LNG).

Low pressure mixed refrigerant stream 545 exiting the MCHE 546 iscompressed in the low pressure refrigerant compressor 554 to producestream 555. Stream 555 is cooled in intercooler 556 to produce stream557. Stream 557 is further compressed in high pressure refrigerantcompressor 558 to produce stream 559. Stream 559 is cooled inaftercooler 564 to be recycled back as original mixed refrigerant stream504.

Power is supplied to the refrigeration compressors 518, 526, 554, 558,594, 598 by three equal sized directly connected gas turbines 580, 582,592. As illustrated in FIGS. 1, 3, and 4, the gas turbines may includestarter/helper electric motors (not shown in this embodiment) to assistin starting the gas turbines and optimally, to provide additional powerto assist the gas turbines, or for exportation into the power grid whenexcess power is available from the gas turbines.

FIG. 6 illustrates yet another exemplary embodiment 600 as applied toanother three loop refrigeration system. In this exemplary embodiment600, unit 606 precools the natural gas feed stream 602 only. Like unit106 of FIG. 1, unit 606 may comprise a series of heat exchangers,valves, and separators as illustrated in FIG. 2A. Natural gas feedstream 602 is cooled by indirect heat exchange to ultimately producecooled stream 650. The evaporation of propane at the four pressuresresults in four propane vapor streams 610, 612, 614, 616 that may thenbe compressed in compressor 618. The resulting compressed stream 620 maythen be condensed in propane condenser 622, producing liquid stream 624for reintroduction into the series of propane evaporators as shown inFIG. 2A.

In this exemplary embodiment, both mixed refrigerant streams 603, 604are cooled in unit 608. Unit 608 may also comprise a series of heatexchangers, valves, and separators as illustrated in FIG. 2B. The mixedrefrigerant streams 603, 604 may also be cooled by indirect heatexchange to ultimately produce cooled streams 605, 638. The evaporationof propane at the four pressures results in four propane vapor streams630, 632, 634, 636 that may then be compressed in compressor 626. Theresulting compressed stream 627 may then be condensed in propanecondenser 628, producing liquid stream 629 for reintroduction into theseries of propane evaporators as shown in FIG. 2B.

Cooled mixed refrigerant stream 638 is subcooled in the cryogenic heatexchanger (MCHE) 646 producing stream 647. Stream 647 may then bereduced in pressure through isenthalpic valve 648 producing stream 649.Stream 649 may then be vaporized in the shell side of the MCHE 646 toprovide cooling to tubeside streams 605, 638, and 650.

Cooled mixed refrigerant stream 605 may also be subcooled and liquefiedin MCHE 646 producing stream 669 then subcooled in exchanger 668producing stream 651. Exchanger 668 may be a wound coil type exchanger,for example. The resulting stream 651 may then be reduced in pressurethrough isenthalpic valve 652 to produce stream 653. Stream 653 may thenbe vaporized in exchanger 668 to provide refrigeration for subcoolingboth the feed gas stream (entering as stream 667 and exiting as 666) andthe third refrigerant stream 669. After vaporization and warming, thirdrefrigerant stream 653 exits exchanger 668 as stream 693 and is thencompressed by compressor 694 to produce stream 695. Stream 695 is thencooled in the mixed refrigerant intercooler 696 to produce stream 697.Stream 697 is compressed in compressor 698 to produce stream 699. Stream699 is then cooled in mixed refrigerant aftercooler 601 to be recycledback as original stream 603.

The cooled natural gas feed stream 650 may enter the MCHE 646 where itis further cooled producing stream 667. Stream 667 may then be subcooledin exchanger 668 to produce product stream 666 that may be, for example,liquid natural gas (LNG).

Low pressure mixed refrigerant stream 645 exiting the MCHE 646 iscompressed in the low pressure refrigerant compressor 654 to producestream 655. Stream 655 is cooled in intercooler 656 to produce stream657. Stream 657 is further compressed in the high pressure refrigerantcompressor 658 to produce stream 659. Stream 659 is cooled inaftercooler 664 to be recycled back as original mixed refrigerant stream604.

Power is supplied to the refrigeration compressors 618, 626, 654, 658,694, 698 by three equal sized directly connected gas turbines 680, 682,692. As illustrated in FIGS. 1, 3, and 4, the gas turbines may includestarter/helper electric motors (not shown in this embodiment) to assistin starting the gas turbines and optimally, to provide additional powerto assist the gas turbines, or for exportation into the power grid whenexcess power is available from the gas turbines.

FIG. 7A illustrates another exemplary embodiment 700A as applied to yetanother three loop refrigeration system. In this exemplary embodiment700A, unit 706 precools the natural gas feed stream 702 and the mixedrefrigerant stream 704. Like unit 106 of FIG. 1, unit 706 may comprise aseries of heat exchangers, valves, and separators as illustrated in FIG.2A. Natural gas feed stream 702 and mixed refrigerant stream 704 iscooled by indirect heat exchange to ultimately produce cooled streams750, 738. The evaporation of propane at the four pressures results infour propane vapor streams 710, 712, 714, 716 that may then becompressed in compressor 718. The resulting compressed stream 720 maythen be condensed in propane condenser 722, producing liquid stream 724for reintroduction into the series of propane evaporators as shown inFIG. 2A.

In this exemplary embodiment, only mixed refrigerant stream 703 iscooled in unit 708. Unit 708 may also comprise a series of heatexchangers, valves, and separators as illustrated in FIG. 2B. The mixedrefrigerant stream 703 is cooled by indirect heat exchange to ultimatelyproduce cooled streams 705. The evaporation of propane at the fourpressures results in four propane vapor streams 730, 732, 734, 736 thatmay then be compressed in compressor 726. The resulting compressedstream 727 may then be condensed in propane condenser 728, producingliquid stream 729 for reintroduction into the series of propaneevaporators as shown in FIG. 2B.

Cooled mixed refrigerant stream 738 is subcooled in the cryogenic heatexchanger (MCHE) 746 producing stream 747. Stream 747 may then bereduced in pressure through isenthalpic valve 748 producing stream 749.Stream 749 may then be vaporized in the shell side of the MCHE 746 toprovide cooling to tubeside streams 705, 738, and 750.

Cooled mixed refrigerant stream 705 may also be subcooled and liquefiedin MCHE 746 producing stream 769 then subcooled in exchanger 768producing stream 751. Exchanger 768 may be a wound coil type exchanger,for example. The resulting stream 751 may then be reduced in pressurethrough isenthalpic valve 752 to produce stream 753. Stream 753 may thenbe vaporized in exchanger 768 to provide refrigeration for subcoolingboth the feed gas stream (entering as stream 767 and exiting as 766) andthe third refrigerant stream 769. After vaporization and warming, thirdrefrigerant stream 753 exits exchanger 768 as stream 793 and is thencompressed by compressor 794 to produce stream 795. Stream 795 is thencooled in the mixed refrigerant intercooler 796 to produce stream 797.Stream 797 is compressed in compressor 798 to produce stream 799. Stream799 is then cooled in mixed refrigerant aftercooler 701 to be recycledback as original stream 703.

The cooled natural gas feed stream 750 may enter the MCHE 746 where itis further cooled producing stream 767. Stream 767 may then be subcooledin exchanger 768 to produce product stream 766 that may be, for example,liquid natural gas (LNG).

Low pressure mixed refrigerant stream 745 exiting the MCHE 746 iscompressed in the low pressure refrigerant compressor 754 to producestream 755. Stream 755 is cooled in intercooler 756 to produce stream757. Stream 757 is further compressed in the high pressure refrigerantcompressor 758 to produce stream 759. Stream 759 is cooled inaftercooler 764 to be recycled back as original mixed refrigerant stream704.

Power is supplied to the refrigeration compressors 718, 726, 754, 758,794, 798 by three equal sized directly connected gas turbines 780, 782,792. As illustrated in FIGS. 1, 3, and 4, the gas turbines may includestarter/helper electric motors (not shown in this embodiment) to assistin starting the gas turbines and optimally, to provide additional powerto assist the gas turbines, or for exportation into the power grid whenexcess power is available from the gas turbines.

FIG. 7B illustrates yet another exemplary embodiment 700B similar to700A, however, in this exemplary embodiment 700B, unit 706 precools thenatural gas feed stream 702 and the mixed refrigerant stream 704 throughindirect heat exchange with a mixed refrigerant stream in a two-stagemixed refrigerant precooling system. While FIG. 7B discloses use of atwo-stage mixed refrigerant precooling system, the precooling may beperformed using a single-stage mixed refrigerant precooling system, ormixed refrigerant precooling systems with greater than two stages, forexample. Additionally, a mixed refrigerant precooling system may beinterchanged with the propane precooling systems disclosed in any of theexemplary embodiments.

FIGS. 8A and 8B illustrate exemplary units 706 and 708 shown in FIG. 7B.Unit 706 may comprise two heat exchangers 810, 812 where streams 702,704, and at least a portion of stream 724 are cooled through indirectheat exchange against stream 713 in heat exchanger 810. Stream 724enters heat exchanger 810 and is cooled producing stream 830. Stream 830is split into two streams 831, 832 where stream 831 is further cooled inheat exchanger 812 while stream 832 is let down in pressure acrossisenthalpic valve 814 to produce stream 833. Stream 833 then enters heatexchanger 810 to provide cooling to streams 702, 704, 724 and exits theheat exchanger 810 as stream 713.

After stream 831 is cooled in heat exchanger 812 to produce stream 834and let down in pressure across isenthalpic valve 816, the resultingstream 835 is introduced into heat exchanger 812 to provide furthercooling for resultant streams 738, 750, 834.

Unit 708 may comprise two heat exchangers 818, 820 where streams 703,729 are cooled through indirect heat exchange against stream 733 in heatexchanger 818. Stream 729 enters heat exchanger 818 and is cooledproducing stream 840. Stream 840 is split into two streams 841, 842where stream 841 is further cooled in heat exchanger 820 while stream842 is let down in pressure across isenthalpic valve 822 to producestream 843. Stream 843 then enters heat exchanger 818 to provide coolingto streams 703, 729 and exits the heat exchanger 818 as stream 733.

After stream 841 is cooled in heat exchanger 820 to produce stream 844and let down in pressure across isenthalpic valve 824, the resultingstream 845 is introduced into heat exchanger 820 to provide furthercooling for resultant streams 705, 844.

Heat exchangers 810, 812, 818, 820 may be wound-coil heat exchangers,plate-and-fin brazed aluminum (core) type heat exchangers, or shell andtube heat exchangers, for example. Heat exchangers 810, 812 may becombined into a single heat exchanger, for example. Heat exchangers 818,820 may also be combined into a single heat exchanger, for example.Finally, heat exchangers 810, 812, 818, 820 may be combined into asingle heat exchanger, for example. Heat exchangers 810, 812, 818, 820may accept two or more load streams, for example.

Pre-cooling in units 106, 108 may provide, for example, enough coolingto feed stream 102 and liquefaction refrigerant stream 104 such that thetemperatures of streams 150 and 138 may reach +60° F. to as low as −100°F. before further cooling in the MCHE 146. The same cooling ranges maybe achieved in FIGS. 3-7B. In one embodiment, for example, propane maybe used as the pre-cooling refrigerant to reach the temperature range of+20° F. to −40° F.

The isenthalpic valves 148, 152 (and the corresponding isenthalpicvalves in FIGS. 3-7B) may optionally be replaced by work extractingliquid turbines, for example, to improve efficiency. Additionally,propane condensers 122, 128 (and the corresponding propane condensers inFIGS. 3-7A) may be ambient heat sink coolers used to condense,desuperheat, and/or optimally subcool precooling refrigerant, forexample.

EXAMPLE

The following example is based on a computer simulation of FIGS. 1, 2A,and 2B as applied to a propane precooled mixed refrigerant process. Asin FIG. 1, the natural gas feed stream 102 entered unit 106 afterpretreatment, including the removal of moisture (H₂O), carbon dioxide(CO₂), sulfur dioxide (SO₂), mercury, and other heavy components,including, but not limited to, benzene, ethylbenzene, and toluene, ifthey exist in the natural gas feed stream 102 in concentrations thatwould lead to freezing in the MCHE 146. The pretreated natural gas feedstream 102 was at 35° C. and 40 bar absolute and had a flow rate of12,260 kg-mole/hr. Natural gas feed stream 102 was cooled by indirectheat exchange in a series of propane evaporators 202, 204, 206, 208(illustrated in FIG. 2A) that operate at successively lower pressures of7.16 bar, 4.25 bar, 2.54 bar and 1.47 bar, where propane evaporator 202is at the highest pressure and propane evaporator 208 is at the lowestpressure. The evaporation of propane at the four pressures resulted infour propane vapor streams 110, 112, 114, 116 that were then compressedin compressor 118. Resulting stream 120 (at 16.2 bar, and 10,930kgmole/hr) was then condensed in propane condenser 122 using an ambientheat sink (air or water), producing liquid stream 124.

The natural gas feed stream 102 was precooled by the propane to −22.5°C. Resulting cooled stream 150 was then cooled and liquefied in MCHE 146by vaporizing mixed refrigerant producing liquid natural gas (LNG)stream 166 at −163.3° C.

The mixed refrigerant stream 104 had a molar composition as follows:

TABLE I Component Mole Composition (%) Nitrogen 12 Methane 38 Ethane 42Propane 8

The mixed refrigerant stream 104 was at 35° C. and 62 bar absolute andhad a flow rate of 50,250 kg-mole/hr. The mixed refrigerant stream 104was cooled by indirect heat exchange in a series of propane evaporators222, 224, 226, 228 (illustrated in FIG. 2B) that operate at successivelylower pressures of 7.16 bar absolute, 4.25 bar, 2.54 bar and 1.47 barwhere propane evaporator 203 is the highest and propane evaporator 209is the lowest. The evaporation of propane at the four pressures resultsin four propane vapor streams 130, 132, 134, 138 which are thencompressed in compressor 126. Resulting stream 127 (at 16.2 bar absoluteand 31,600 kgmole/hr) is condensed in propane condenser 128 using anambient heat sink (air or water), producing liquid stream 129.

The precooled mixed refrigerant stream 138 is then separated into liquidstream 142 and vapor stream 144 in phase separator 140. Liquid stream142 is then subcooled to −125° C., flashed isenthalpically through valve148, and then vaporized in the shell side of exchanger 146 to providecooling to the tubeside streams 142, 144, 150. Vapor stream 144 isliquefied, subcooled to a temperature of −163° C., flashedisenthalpically through valve 152, and then vaporized and warmed in theshell side of exchanger 146 to provide cooling to the tubeside streams142, 144, 150. After vaporization and warming, the combined mixedrefrigerant stream 145 exits the MCHE 146 at a temperature of −32.7° C.and a pressure of 4.14 bar absolute. The combined mixed refrigerantstream 154 is then compressed in three stages of compressors 156, 158,160 back to a pressure of 62 bar absolute, completing the loop.

Comparison with U.S. Pat. No. 6,962,060

Computer simulations of the exemplary embodiment illustrated in FIG. 1were performed on the same basis as the simulation of a propaneprecooled mixed refrigerant process utilizing the precooling arrangementof U.S. Pat. No. 6,962,060.

Results for the simulations are listed in Table II below. For bothsimulations, the same propane low pressure suction pressure was assumedand two compressor casings were required. For both simulations,preliminary sizing calculations for the compressors were performed. Inthe case of the exemplary embodiment illustrated in FIG. 1, thecompressor casings 118 and 126 were smaller in diameter and had lowervolumetric flow rates translating into lower cost. In addition,depending on the vendor and the scale of the plant, construction oflarge diameter impellers and casings may not have been feasible, thus,the solution utilizing the prior art may have been more limited inscale-up potential.

As illustrated in Table II, the exemplary embodiment of FIG. 1 allowsmore optimal and feasible compressor designs than the system disclosedin U.S. Pat. No. 6,962,061 using the same number of compressor casingsand providing the same pre-cooling service. This is achieved bysegregating the heat loads requiring pre-cooling refrigeration into twoindependent systems.

TABLE II U.S. Pat. Exemplary No. Embodiment in 6,962,060 FIG. 1Precooling −30.2 −30.2 Temperature (° C.) Liquid Natural Gas 490,000490,000 Production (kg/h) Compressor 1 Identifier Compressor 43Compressor 126 Maximum Impeller 55 50 Diameter (inches) Maximum Volume149,000 119,000 Flow Rate (m³/hr) Compressor 2 Identifier Compressor 49Compressor 118 Maximum Impeller 52 51 Diameter (inches) Maximum Volume78,000 57,000 Flow Rate (m³/hr)

While aspects of the present invention has been described in connectionwith the preferred embodiments of the various figures, it is to beunderstood that other similar embodiments may be used or modificationsand additions may be made to the described embodiment for performing thesame function of the present invention without deviating therefrom.Therefore, the claimed invention should not be limited to any singleembodiment, but rather should be construed in breadth and scope inaccordance with the appended claims.

1. A natural gas liquefaction system, the system comprising: a firstprecooling refrigeration system that accepts at least a natural gas feedstream; a second precooling refrigeration system that accepts at least afirst refrigerant stream; and a cryogenic heat exchanger fluidlyconnected to the first precooling refrigeration system and the secondprecooling refrigeration system that accepts the natural gas feed streamfrom the first precooling refrigeration system and the first refrigerantstream from the second precooling refrigeration system to liquefy thenatural gas feed stream, wherein the second precooling refrigerationsystem accepts only stream(s) having a composition different from thestream(s) accepted by the first precooling refrigeration system.
 2. Thesystem of claim 1, wherein the first refrigerant stream is a mixedrefrigerant stream.
 3. The system of claim 1, wherein the firstrefrigerant stream comprises nitrogen, methane, ethane, and propane. 4.The system of claim 1, further comprising a subcooler exchanger fluidlyconnected to the cryogenic heat exchanger, wherein the subcoolerexchanger accepts a second refrigerant stream from the cryogenic heatexchanger to subcool the natural gas feed stream through indirect heatexchange.
 5. The system of claim 1, wherein the first precoolingrefrigeration system and the second precooling refrigeration system eachcomprise: at least one propane evaporator; and a propane compressorfluidly coupled to the at least one propane evaporator and adapted toaccept at least one propane vapor stream.
 6. The system of claim 1,wherein the first precooling refrigeration system and the secondprecooling refrigeration system are CO₂ refrigeration systems.
 7. Thesystem of claim 1, wherein the first precooling refrigeration systemcomprises at least one heat exchanger that accepts at least two loadstreams.
 8. The system of claim 5, further comprising a first driver anda second driver, wherein the first driver drives the propane compressorof the first precooling refrigeration system, the propane compressor ofthe second precooling refrigeration system; and a first high pressurerefrigerant compressor, and wherein the second driver drives a firstmedium pressure refrigerant compressor and a first low pressurerefrigerant compressor.
 9. The system of claim 5, further comprising afirst driver and a second driver, wherein the first driver drives thepropane compressor of the first precooling refrigeration system and afirst low pressure refrigerant compressor, and wherein the second driverdrives the propane compressor of the second precooling refrigerationsystem and a first high pressure refrigerant compressor.
 10. The systemof claim 5, further comprising a first driver and a second driver,wherein the first driver drives the propane compressor of the firstprecooling refrigeration system and the propane compressor of the secondprecooling refrigeration system and the second driver drives a first lowpressure refrigerant compressor and a first high pressure refrigerantcompressor.
 11. The system of claim 10, further comprising a thirddriver, wherein the third driver drives a second low pressurerefrigerant compressor and a second high pressure refrigerantcompressor.
 12. The system of claim 8, wherein the first driver and thesecond driver are gas turbines.
 13. The system of claim 1, wherein thecryogenic heat exchanger is a wound-coil heat exchanger.
 14. A methodfor liquefying natural gas, the method comprising the steps of:providing a natural gas feed stream; providing a first refrigerantstream; precooling in a first precooling refrigeration system at leastthe natural gas feed stream; precooling in a second precoolingrefrigeration system at least the first refrigerant stream; andvaporizing the precooled first refrigerant stream in a cryogenic heatexchanger to cool the precooled natural gas feed stream through indirectheat exchange, wherein the second precooling refrigeration systemprecools only stream(s) having a composition different from thestream(s) precooled by the first precooling refrigeration system. 15.The method of claim 14, wherein the natural gas feed stream and thefirst refrigerant stream are precooled to +60° F. to −100° F.
 16. Themethod of claim 14, further comprising providing a second refrigerantstream, wherein the second refrigerant stream is precooled in the firstprecooling refrigeration system or the second precooling refrigerationsystem and is vaporized to subcool the natural gas feed stream.
 17. Themethod of claim 14, wherein the first refrigerant stream is a mixedrefrigerant stream.
 18. A natural gas liquefaction system for largecapacity liquefaction plants, the system comprising: a first precoolingrefrigeration system that accepts one stream selected from the groupconsisting of: a natural gas feed stream, and an at least onerefrigerant stream; a second precooling refrigeration system thataccepts any remaining stream(s) not accepted by the first precoolingrefrigeration system and from the group consisting of: the natural gasfeed stream, and the at least one refrigerant stream; and a cryogenicheat exchanger fluidly connected to the first precooling refrigerationsystem and the second precooling refrigeration system and adapted toaccept the natural gas feed stream and the at least one refrigerantstream from the first precooling refrigeration system and the secondprecooling refrigeration system, wherein the at least one refrigerantstream is used to liquefy the natural gas feed stream, wherein thesecond precooling refrigeration system accepts only stream(s) having acomposition different from the stream(s) accepted by the firstprecooling refrigeration system.
 19. The system of claim 18, wherein theat least one refrigerant stream is a mixed refrigerant stream.
 20. Thesystem of claim 18, wherein the at least one refrigerant streamcomprises a first refrigerant stream and a second refrigerant stream.